The present invention relates to static relays and more particularly to a novel dual-comparator and clamp circuit for simplified linear adjustment of the static relay time delay period over a multiple order of magnitude range without altering the basic charging curve of the delay network.
The present invention is an improvement of the static relays described and claimed in U.S. Pat. Nos. 3,319,127, issued May 9, 1967; 3,573,555, issued Apr. 6, 1971; and 3,766,436, issued Oct. 16, 1973, all of which are assigned to the assignee of the present invention.
The above-indicated patents describe static relay devices having an inverse time-current characteristic and which replace electromechanical relays which were previously employed to obtain the desired characteristic. The static relay devices of the above patents essentially comprise a pick-up circuit which is connected to the circuit being monitored; a timing circuit which is charged from the pick-up circuit when the circuit being monitored develops or otherwise generates an output which exceeds a known threshold value according to a predetermined inverse current-time characteristic so as to activate the pick-up circuit; and a tripping circuit which is operated in accordance with the inverse time-current relationship of the timing circuit so as to be tripped when the output of the timing circuit reaches the tripping threshold of the tripping circuit. The characteristic of the timing circuit is related to the circuit being monitored such that when the condition of the circuit being monitored exceeds a predetermined value over time intervals associated therewith a tripping operation is performed. It should be understood, in this respect, that overcurrent conditions of small magnitude must persist for a greater time interval to initiate a tripping operation and conversely overcurrent conditions of large magnitude will cause faster time-out of the timing circuit to cause initiation of the tripping operation.
It is desirable to provide such static relays with the capability of being adjusted over extremely large time delay ranges of several orders of magnitude, e.g., 1000:1 or 10,000:1.
One technique, described in U.S. Pat. No. 3,573,555, utilizes a variable duty-cycle free-running multivibrator circuit powered by the line being monitored, having "on" and "off" time periods which are adjustable over a substantial time range without altering the desired curve shape of the time-current characteristic. This adjustment, in accordance with an adjustable resistance element and a time dial setting knob provided therewith, resulted in an impractical and non-uniform inverse marking of the potentiometer dial and was extremely complex and tedious to design, assemble, and use. A later technique, described in U.S. Pat. No. 3,766,436, utilizes a variable duty-cycle free-running multivibrator circuit powered by the line being monitored, having a substantially constant "off" time interval and adjustable "on" time intervals to provide a substantially linear adjustable time range extension ratio and time dial therefor.
However, the variable duty-cycle clamp circuit of U.S. Pat. No. 3,766,436 has a maximum adjustment range of approximately fifteen times the base charging rate of the basic time delay network. To obtain a 5% unit-to-unit repeatability of the time extension previously realized by the use of this multivibrator, the circuit of the last-mentioned patent requires closely matched transistors and capacitors, which matching procedure is extremely tedious and costly.